Modification of a plant trait through genetic engineering depends upon the insertion into the plant genome of a polynucleotide construct containing the gene of interest, operably linked to a promoter that is functional in the transgenic plant. Within a plant genome, any single gene is, in general, operably linked to a promoter that will determine when and where, within the plant tissues and organs, the gene should be expressed. Sometimes, it is of interest to use a promoter capable of directing the expression of the operably linked gene to most tissues of the plant. These promoters are known in the art as constitutive promoters. To be most useful, a constitutive promoter should be able to direct the expression to all cells, tissues and organs of the plant. Constitutive promoters should also preferably be able to determine the expression of the operably linked gene to the same high level in all tissues and organs, throughout the plant's life cycle. Therefore if one wants to express a gene of interest in several or all tissues or organs within a transgenic plant, constitutive promoters must be used.
In a number of situations the expression of particular genes in most or all tissues or organs confers a phenotype of interest to the plant. For example, if one wants to improve the plant's disease resistance, a gene that confers such phenotype linked to a constitutive promoter is inserted, rather than using tissue-specific promoters that would allow the gene to be expressed in selected plant tissues, causing in some cases undesirable phenotypes.
Thus far, the production of genetically engineered plants expressing useful and/or desirable traits requires the availability of promoters that permit the gene or genes of interest to be expressed constitutively. Thus, isolation and characterization of constitutive promoters that can serve as regulatory regions for expression of heterologous nucleotide sequences of interest in most or all tissues and organs is essential for the genetic engineering of plants.